Regulation of pollutant discharges from municipal wastewater treatment systems has become more stringent in recent years. In response, many municipalities have deployed new wastewater treatment systems or retrofitted existing systems to reduce pollutant discharge. Pollutants can be of many forms with the most common being Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Suspended Solids (TSS), ammonia, total nitrogen, nitrate, nitrite and phosphorous.
Biological treatment systems such as conventional activated sludge systems and membrane bioreactors are one method to reduce the pollutants in a wastewater influent. Biological treatment systems are designed and operated to retain an adequate amount of activated sludge such that the pollutant load contained in the water treated by the system will be adequately reduced. The amount of activated sludge is related to the Solids Retention Time (SRT) of the system, and the minimum SRT required to treat various pollutants under various conditions is generally well known. Conventional activated sludge systems retain activated sludge by the use of settling or clarification devices and can maintain adequate SRTs to treat pollutants provided that both the flow and the activated sludge concentration to the settling basins or clarifiers are within reasonable limits, which depend upon the area of the settling basins or clarifiers and the characteristics of the activated sludge. Membrane bioreactor systems retain the activated sludge by the use of membrane filtration equipment and can operate successfully at significantly higher activated sludge concentrations than typical for conventional activated sludge systems, but are more limited in their ability to process occasional high flow rates.
The flow rate and pollutant load of an influent treated by a municipal wastewater system, which may include industrial wastewater, residential wastewater, and precipitation runoff, can vary significantly over time. In addition to normal diurnal variations, significant rain events can cause short-term spikes in wastewater influent flow rate and pollutant load. Several systems have been developed that can accommodate significantly varying flow rates and pollutant loads, including membrane bioreactors in combination with flow equalization storage tanks, membrane bioreactors with additional membranes for the treatment of peak flows, conventional activated sludge systems with parallel chemical treatment systems, and conventional activated sludge systems with oversized tanks and clarifiers. Each of these systems for accommodating significantly varying flow rates and pollutant loads significantly increases the operating costs and land area requirements for a wastewater treatment plant, and those systems utilizing parallel chemical treatment systems do not produce as high a quality of effluent.
Varying flow rates and pollution loads are just two factors that must be considered when designing a wastewater treatment system. The characteristics of the wastewater, including its temperature and the types of pollutants that it contains, are another. Many biological treatment systems employ two types of biological material to reduce a wastewater's ammonia, organic material, and nitrate concentrations: autotrophic organisms, which are also called “nitrifiers,” are used to convert ammonia to nitrate and heterotrophic organisms are used to remove organic materials and nitrates. The growth rates of “nitrifiers” are usually much lower than those of the heterotrophic organisms. Moreover, the wastewater temperature can significantly impact the growth rate of nitrifiers. See, e.g., Grady, et al., Biological Wastewater Treatment, Second Edition, Marcel Dekker, N.Y. (1999).
In northern climates, winter wastewater temperatures are sometimes 10° C. or lower. To ensure sufficient nitrification to meet discharge requirements during winter months, wastewater treatment systems in such climates are typically designed with solids retention times of 8 days or longer. Longer solids retention times require systems with a larger fluid capacity. Increasing the solids retention time in conventional biological treatment systems also increases the mixed liquor suspended solids concentrations, requiring larger-capacity aeration systems and secondary clarifiers, which tends to increase expenses for a wastewater treatment plant because of factors such as higher permitting and operating costs and greater land area requirements.
As an alternative to larger aeration systems and secondary clarifiers, some systems achieve good nitrogen removal at low solids retention times by nitrifier bioaugmentation, in which nitrifiers are added from a separate seed source. For example, wastewater treatment systems that include certain trickling filters/activated sludge processes can partially nitrify ammonia in the trickling filter. See, e.g., Daigger, et al., “Process and Kinetic Analysis of Nitrification in Coupled Trickling Filter/Activated Sludge Processes,” Water Environment Research, Vol. 65, pp. 679-685 (1993). Nitrifiers grown in the trickling filter are allowed to slough off the filter to “seed” the activated sludge process, enabling stable nitrification at decreased solids retention times in the suspended growth bioreactor.
Another example of nitrifier bioaugmentation is described by Constantine in a 1996 Masters thesis at McMasters University entitled “Bioaugmentation to Achieve Nitrification in Activated Sludge Systems.” Two parallel sequencing batch reactors are operated at two different solids retention times. One sequencing batch reactor is operated at a solids retention time long enough to ensure thorough nitrification; this reactor is called the “donor” reactor. The other sequencing batch reactor is operated at a solids retention time too short to allow significant nitrification; this reactor is called the “receiver” reactor. Waste activated sludge is directed from the donor to the receiver reactor, resulting in a constant supply of nitrifiers from the donor to the receiver reactor. This allows significant nitrification to occur in the receiver reactor at solids retention times less than those required absent the bioaugmentation. (See also Constantine's U.S. Pat. No. 6,723,244.)
Tendaj-Xavier proposed a two-stage wastewater treatment process in a 1983 dissertation at the Royal Technical University entitled “Biological Treatment of Sludge Water from Centrifugation of Digested Sludge.” The dissertation generally suggests growing nitrification bacteria on the dewatering centrate produced within a wastewater plant and seeding the nitrified bacteria into the primary wastewater stream. However, Tendaj-Xavier observes that the high initial capital expense and/or increased space requirements of the proposed system could be prohibitive.
The process described in U.S. Pat. No. 5,811,009 (Kos) also relies on bioaugmentation. See also P. Kos, “Short SRT (Solids Retention Time) Nitrification Process/Flowsheet,” Wat. Sci. Tech., Vol. 38, No. 1, pp. 23-29 (1998). The Kos process configuration mitigates some drawbacks of the process proposed by Constantine (1996). Kos' system adds a sidestream reactor, which is distinct from the main waste treatment stream, to treat recycle streams from anaerobic digesters. These recycle streams are rich in ammonia, which is released during anaerobic digestion, and support an enriched culture of nitrifiers in the sidestream reactor. The temperature of the recycle streams is often elevated, which promotes nitrification. Kos' steady-state simulations suggested that the described process allowed nitrification of the main waste treatment stream at reduced solids retention times.
The Kos process configuration has potential problems, especially with respect to the sidestream plant operation. For example:                This system may require high supplementary alkalinity to maintain process stability.        Substrate and product inhibition in this process may render the process unstable, as described in Anthonisen, et al., “Inhibition of nitrification by ammonia and nitrous acid,” Journal of the Water Pollution Control Federation, Vol. 48, pp. 835-852 (1976).        The poor settling characteristics of enriched nitrifier cultures may interfere with maintaining consistent solids retention times in the side stream reactor. See, e.g., “U.S. Environmental Protection Agency, Process Design Manual for Nitrogen Control,” EPA/625/R-93/010, U.S. Environmental Protection Agency, Cincinnati, Ohio (1993).        
The entirety of each of these patents and other publications, and of any patents or other publications referred to below, is incorporated herein by reference.